Physics of Laser Cutting
Laser Machining ProcessesSeveral major laser machining processes are illustrated below. (The notable omission is welding, beyond the scope of this document.)
This diagram illustrates the cutting process in cross section. Note that a gas assist can be used to speed cutting of metals.
Drilling RatesA simple method of analysis is proposed in (Wilson, 1987). The energy to vaporize a mass m of solid material at intial temperature T is
Usually, L_{f} << L_{v} and T << T_{v} , and C_{s} » C_{L} = C. We then have the simple form E_{v} = m(CT_{v}+L_{v}). Now, consider a circular laser beam of area A boring into the surface of such a material with a velocity v_{s }directed into the material. It must remove a section of mass v_{sr }A per unit time. Ignoring reflectance from the material surface, the heat flow is equal to the beam power P. Assuming a beam with diameter d and equal power over A, we have P = v_{s}(p d^{2}/4)(CT_{v}+L_{v}). If v_{s} exceeds the normal rate of heat diffusion into the material, this equation is fairly accurate for estimating drilling rates or hole depths. To find hole depths, solve for the quantity v_{s}t, where t is the duration of the beam pulse. For example, consider a 100msec pulse from a 10W laser with a beam diameter of 1mm. If this were to strike a Perspex (methyl methacrylate) sheet, the resultant hole would have a depth of 1.6mm. Note the inversesquare dependence of hole depth on beam diameter. Halving the beam diameter results in a hole four times deeper. This highlights the importance of beam focusing in laser machine design.
Cutting RatesThis model can be used to estimate cutting rates as well. Consider the laser scanning over the surface of the material with velocity v_{b}. As it scans, it cuts through the material to a depth z = v_{s}d/v_{b}. We now have
The following table can then be used to approximate the laser power necessary to cut a given material.
(Chryssolouris, 1991) describes a model that accounts for the material absorptivity as well.
Note the following:
